The present invention relates generally to output devices for integrated circuits, and more particularly to output devices having parasitic transistors for increased current drive.
The data rates required of integrated circuit inputs and outputs has increased dramatically the past few years, and the demand for ever higher data rates shows no signs of abating. Higher data rates mean that capacitances at device input and output pins need to be driven more quickly; this requires higher output currents to be provided by output devices.
Process changes can provide part of the solution. For example, as transistor gate lengths become smaller, the current driving capability of a device is increased. But for a specific technology or process, increasing the drive capability of an output device becomes difficult.
Conventional techniques used to increase current drive are limited by various tradeoffs. For example, the size of an output device can be increased. However, the use of a larger device increases output capacitance, which diminishes the gain in performance that would otherwise be achieved. Also, the sizes of devices driving the gate of the output device must be scaled larger when an output device's size is increased, thereby increasing power dissipation.
Another conventional solution is to increase the voltage amplitude of the signal driving the output device. Providing a higher gate-to-source voltage to an output device increases its output current. But power dissipation is a major concern in modern integrated circuits. Accordingly, device power supply voltages have been decreasing over time, reducing the available voltage drive. In any event, a higher voltage drive can lead to oxide problems in the output device caused by hot-carrier injection or time-dependent dielectric breakdown.
Accordingly, what is needed are circuits, methods, and apparatus that provide output devices having an increased drive capability. These devices can be used to provide higher currents for a given device size. These devices could also be used to reduce device sizes while maintaining a certain drive capability. Such output devices could be used to provide a more efficient output device, that is, an output device that is capable of providing a larger output current for a given die area.
Accordingly, embodiments of the present invention provide circuits, methods, and apparatus for output devices having parasitic transistors for a higher output current drive capability.
An exemplary embodiment of the present invention provides a MOS output device having a parasitic bipolar transistor that assists output voltage transitions. The parasitic transistor may be inherent in the structure of the MOS device. Alternately, one or more regions, such as implanted or diffused regions, may be added to the MOS device to form or enhance the parasitic bipolar device. The parasitic transistor is turned on during an appropriate output transition and turned off once the transition is complete. The parasitic device may be turned on by injecting current into the bulk of a pull-down device, by pulling current out of the bulk of a pull-up device, or by tying the bulk of the output device to an appropriate voltage, such as VCC for a pull-down device or ground for a pull-up device.
A specific embodiment of the present invention provides an n-channel pull-down device having a parasitic NPN device. The NPN device is turned on either by injecting base current into the bulk of the MOS pull-down device, or by tying the bulk of the MOS pull-down device to an appropriate voltage such as VCC. The base current is injected during the time after the pull-down device receives a low-to-high voltage transition at its gate and before an output voltage at its drain reaches a low level.
A specific embodiment of the present invention prevents substrate leakage currents by tying the bulk of the MOS device to its source during at least some of the time that base current is not being injected into its bulk. Various embodiments of the present invention may incorporate one or more of these and the other features described herein.
A better understanding of the nature and advantages of the present invention may be gained with reference to the following detailed description and the accompanying drawings.